Techniques for measurement of picomole amounts of ammonium ion or ammonia are being explored. One highly sensitive approach involves the use of sensitized mercury luminescence. This approach has good sensitivity but quenching of the luminescence gives problems with reproducibility. Both oxygen and water vapor quench significantly and must be excluded. An alternative technique, an enzymatic method involving the conversion of ammonium ion and alpha-ketoglutarate to glutamic acid coupled to the fluorometric detection of changes in NADH offers good sensitivity and specificity. This technique requires the use of a new micro-fluorometer, described elsewhere.